Render not saving correctly
What I think it could have happened is that you have simply disabled the File Extensions checkbox, under the Output Section in the Render Settings. Just reenable it and from the next render, the images will have the right extension.

If you want to be able to see the images you render, insert the extension that you selected in the rendering in the file name. For example with the render settings in the photo, you would have to insert ".PNG" in the rendered image file names.